Evidence for "inter- and intraspecific horizontal genetic transfers" between anciently asexual bdelloid rotifers is explained by cross-contamination

Abstract

Bdelloid rotifers are microscopic invertebrates thought to have evolved for millions of years without sexual reproduction. They have attracted the attention of biologists puzzled by the maintenance of sex among nearly all other eukaryotes. Bdelloid genomes have an unusually high proportion of horizontally acquired non-metazoan genes. This well-substantiated finding has invited speculation that homologous horizontal transfer between rotifers also may occur, perhaps even 'replacing' sex. A 2016 study in Current Biology claimed to supply evidence for this hypothesis. The authors sampled rotifers of the genus Adineta from natural populations and sequenced one mitochondrial and four nuclear loci. For several samples, species assignments were incongruent among loci, which the authors interpreted as evidence of \"interspecific genetic exchanges\". Here, we use sequencing chromatograms supplied by the authors to demonstrate that samples treated as individuals actually contained two or more divergent mitochondrial and ribosomal sequences, indicating contamination with DNA from additional animals belonging to the supposed \"donor species\". We also show that \"exchanged\" molecules share only 75% sequence homology, a degree of divergence incompatible with established mechanisms of recombination and genomic features of Adineta. These findings are parsimoniously explained by cross-contamination of tubes with animals or DNA from different species. Given the proportion of tubes contaminated in this way, we show by calculation that evidence for \"intraspecific horizontal exchange\" in the same dataset is explained by contamination with conspecific DNA. On the clear evidence of these analyses, the 2016 study provides no reliable support for the hypothesis of horizontal genetic transfer between or within these bdelloid species.